A cyst is a cavity containing liquid, semisolid, or solid material. An epidermoid cyst is a benign cyst that develops out of ectodermal tissue and consists of a thin layer of squamous epithelium. An epidermal cyst may or may not be painful or release pus, and are usually noticed as a bump on the skin. Epidermal cysts and trichilemmal cysts are clinically indistinguishable. The wall of the epidermal cyst conserves the epidermal layers, but the wall of the trichilemmal cysts, which arise from the external root sheath, shows palisading of unlayered epithelial cells. Homogeneous horny material that is further degraded to fat and cholesterol forms within both types of cysts.